Standard Furnace & Air Filter Sizes
The most common furnace filter sizes are 16x25x1, 20x25x1, 16x20x1, 14x25x1, and 20x20x1, plus 4"–5" media sizes like 16x25x4 and 20x25x5. Filters are labelled by nominal size (rounded) but measure about 1/4"–1/2" smaller (actual size). If your size isn't standard, a custom-cut filter ensures a proper fit.
Most common 1" filter sizes
- 16x25x1
- 20x25x1
- 16x20x1
- 14x25x1
- 20x20x1
- 14x20x1
- 12x24x1
- 18x24x1
- 24x24x1
- 15x20x1
Common 4" and 5" media sizes
- 16x25x4
- 20x25x4
- 20x20x4
- 16x25x5
- 20x25x5
- 24x24x4
Nominal vs actual size
Nominal size is the rounded number printed on the filter (e.g. 16x25x1). Actual size is the true measurement, typically slightly smaller (e.g. 15.5 x 24.5 x 0.75). Always order by the nominal size printed on your existing filter, or measure and request the exact actual dimensions for a custom filter.
Frequently Asked Questions
What is the most common furnace filter size?
16x25x1 and 20x25x1 are the two most common residential furnace filter sizes in North America, followed by 16x20x1 and 14x25x1.
What if my filter size isn't standard?
Order a custom-size filter. Modern Air Filtration builds filters to any exact dimension and MERV rating, which is the correct solution for non-standard or older equipment.
How do I read my filter size?
The nominal size is printed on the side of the filter as length x width x depth (e.g. 16x25x1). If it's unreadable, measure the existing filter or the slot and round to the nearest standard size.
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